1. Field of the Invention
The present invention is related to helical scan computed tomography and, in particular, to helical scan computed tomography with improved temporal resolution. The temporal resolution may be improved in each slice image by using an image reconstruction algorithm for multi-slice helical CT which does not need any special signals obtained from a patient.
2. Discussion of the Background
Helical scanning computed tomography (CT) is described, for example, in U.S. Pat. No. 4,630,202. Helical scan CT may incorporate a multi-row detector array to collect data at multiple slice positions, as described in U.S. Pat. No. 4,965,726. Multi-slice helical CT uses such an array. This enables one to improve spatial resolution in the longitudinal direction, to extend the scanning length and to shorten scanning time, which results both in better temporal resolution in volume and better contrast resolution. If the helical scanning is repeated for the same volume, it is possible to observe a temporal variation of the volume.
Various image reconstruction algorithms are used in multi-slice helical CT, such as Algorithm for Image Reconstruction in Multi-Slice Helical CT, Taguchi et al, Med. Phys. 25:550-561 (1998). This algorithm includes a scanning technique consisting of three parts: (1) scanning by optimized sampling scanning whose helical pitch must be carefully selected; (2) interpolation by helical filter interpolation (HFI) which refers to a filtering process in the longitudinal direction, and (3) reconstruction by fan-beam filtered backprojection (a common fan-beam reconstruction technique may be applied in the third step). HFI uses longitudinal filtering for obtaining data at the slice position using either data separated by 180xc2x0 or 360xc2x0, that is, data which is acquired at different time, in the linear interpolation (or extrapolation).
In the optimized sampling scanning, some specific helical pitches (i.e., even integers) should be avoided because the longitudinal data sampling patterns at the central rays are sparse at those helical pitches, which may degrade the image quality in the absence of additional technique. The helical interpolation, HFI, can be described by the following two steps: 1) repeating two-point linear interpolation (or extrapolation) for obtaining plural re-sampled data at positions within xe2x80x9cfilter-widthxe2x80x9d and 2) filtering the re-sampled data. Therefore, HFI results in interpolation or extrapolation, depending on the data sampling positions, when filter width equals zero. Filter width is defined as the ratio of the longitudinal range to the nominal slice thickness.
Special methods have been developed for cardiac imaging with multi-slice helical CT to improve practical temporal resolution, but have some disadvantages. For example, they need an EKG signal to re-sort the data for the same cardiac phase and can only be applied by cyclically moving objects.
In parallel-beam geometry the minimal angle of the required projections for image reconstruction is 180xc2x0. In fan-beam geometry, the projection data spanning 180xc2x0+fan-angle contain such data. However, some projections are measured twice in the projection data and might cause artifacts in the absence of additional process. In order to eliminate the effect of such redundancy and to avoid data truncation, smoothing weight is applied to the projection data sets prior to fan-beam image reconstruction. This algorithm, called half-scanning (HS), has been used to improve the temporal resolution of images in axial CT, as described in Computed Tomography Scanning with Simultaneous Patient Translation, C. Crawford et al., Med. Phys., 17:967-982 (1990).
Other methods that have been used are under-scanning (US), described in Computed Tomography Scanning with Simultaneous Patient Translation, C. Crawford et al., Med. Phys., 17:967-982 (1990). and high temporal resolution reconstruction (HTRR), described in High Temporal Resolution Reconstruction for Reducing Motion Artifact Caused by Cardiac Motion, Shen et al, Jap. J. of Rad. Tech., 54(11): 1287-1294 (1998). The difference among HS, US and HTRR is simply the weighting function, which is shown in FIG. 1. Another method called modified US refers to a weighting technique between xe2x80x9cdirectxe2x80x9d and xe2x80x9ccomplimentaryxe2x80x9d data considering the acquisition timing, and is shown in FIG. 2.
Other techniques, electron beam CT (EBCT) and EKG gated reconstruction in general purpose CT, are used to improve the temporal resolution. EBCT gives the best temporal resolution, 0.05 sec. in its shortest scanning time. However, EBCT has the disadvantages of insufficient longitudinal spatial resolution, system cost, and limited X-ray exposure. Thus, cardiac imaging with multi-slice helical CT, which does not suffer from these problems, has potential value in cardiac imaging.
Recently, EKG gated reconstruction methods with small helical pitch, which sort helical data according to cardiac cycle phase, have been developed for general purpose CT and have shown promise in obtaining better temporal resolution. With helical multi-slice pitch of 1.0, they can achieve 0.06 sec. at FWTM or at FWTA. The biggest advantage of these dedicated cardiac reconstruction methods is the possibility for dynamic volumetric heart imaging during the whole heart cycle. Spatial resolution and image noise may be improved using data separated by 180 and 360 degrees only if their heart phases match. The two biggest problems of this method are that it can be a high dose examination with small helical pitch, and potential problems exist for studies with contrast agents. Since they utilize data obtained at the same cardiac phase but at the different time, changes other than cardiac motion, such as the contrast concentration, can degrade image quality.
The temporal resolution of individual images obtained with methods such as described above are insufficient for rapidly moving organs such as the heart and adjacent pulmonary vessels. There is a need for better temporal resolution in helical CT and in multi-slice helical CT.
It is an object of the present invention to provide a method and system to improve temporal resolution in computed tomography (CT).
It is another object of the invention to provide a method and system to improve temporal resolution in multi-slice helical CT.
It is a further object of the invention to provide improved spatial resolution in CT.
It is yet another object of the invention to provide automatic cardiac volumetric reconstruction in CT.
A still further object of the invention to provide a CT method and system to reconstruct images at different timing.
A yet still further object of the invention is to reduce patient dose.
These and other objects are obtained by a computed tomograph method including steps of exposing a subject to x-rays, collecting data corresponding to an image of the subject, processing the data using at least a portion of data obtained simultaneously, weighting the processing data as a function of data collection timing and reconstructing the image of the subject. The process data may be obtained using only a portion of the data obtained simultaneously. Processing the data may consist of at least one of linear interpolation and extrapolation or at least one of non-linear interpolation and extrapolation.
Complementary data may also be obtained and a weighted summation of the process data and the complementary data may be obtained based on the data collection timing. The image is then reconstructed using the weighted summation.
The weighting of the process data may also be shifted temporally. Also, a signal may be obtained from the subject and a timing for the reconstructing the image may be shifted using the signal. The irradiation of the subject may also be controlled using this signal.
Data may be simultaneously obtained for a plurality of rows of data. Selected ones of the rows of data may be processed to yield the processed data. Data may be selected from the rows closest to portions of a slice being reconstructed for each portion, and the data corresponding to each of the portions may be processed to yield process data. The data may be selected from two of the rows closest to the portions of the slice.
Processing the data may also comprise filtering the data to obtain filtered data, and the weighting may comprise weighting the filter data.
The objects of the invention may also be achieved by computed tomograph system having a radiation source, a detector arranged to receive radiation from the source, a processing circuit for processing data obtained from the detector, a data collection timing weighting circuit connected to the interpolation circuit, and a reconstruction circuit connected to the weighting circuit. The processing circuit may comprise at least one of a non-linear interpolation and an extrapolation circuit, or one of a linear interpolation and an extrapolation circuit. The processing circuit may also comprise a filter circuit. The weighting circuit may comprise means for temporally shifting a weighting function or means for selectively temporally shifting the weighting function.